1. Field of the Invention
The present invention relates to an image-taking apparatus such as a video camera.
2. Description of the Related Art
In recent years, a so-called TV-AF method is mainly used for auto-focus control in video cameras, in which an object image is photoelectrically converted by an image-pickup element into a video signal from which the sharpness of the video is detected and used as an AF evaluation value, and the position of a focus lens at which the AF evaluation value is at the maximum is searched for.
As the AF evaluation value in the TV-AF method, a high-frequency component extracted from a video signal by a band pass filter for a certain band is typically used. When a normal object image is taken, the AF evaluation value is higher as a focus lens approaches an in-focus point (in-focus position) as shown in FIG. 2. The highest level of the AF evaluation value corresponds to the in-focus point.
Another AF method is a TTL (Through The Lens) phase difference detection method. In the TTL phase difference detection method, a luminous flux passing through the exit pupil of an image-taking lens is split into two which are received by a set of line sensors. Then, a displacement amount in signals output according to the received light amounts, that is, a relative positional displacement amount in the direction in which the luminous flux is split, is detected to directly determine a defocus amount of the image-taking lens. Once a focus detection sensor is used to perform storing operation, the drive amount and the drive direction of the focus lens can be provided, so that focus adjustment operation can be performed at high speed.
Another phase difference detection method is an external metering phase difference detection method which does not use light passing through an image-taking lens. In the external metering phase difference detection method, a luminous flux from an object is split into two which are received by a set of line sensors. Then, a displacement amount in signals output according to the received light amounts, that is, a relative positional displacement amount in the direction in which the luminous flux is split, is detected to calculate the distance to the object with triangulation.
Other AF methods using an external metering sensor include a method in which an ultrasonic sensor is used to measure the distance to an object from the propagation speed of ultrasonic waves and a method in which an infrared sensor is used to perform triangulation, and the like.
In addition, a hybrid AF method is used by combining these AF methods. In the hybrid AF method, for example, a focus lens is driven near to an in-focus point in the TTL phase difference detection method, and then the focus lens is driven more accurately to the in-focus position in the TV-AF method (for example, see Japanese Patent Application Laid-Open No. 5-64056 (No. 1993-64056)).
While the TV-AF method generally allows more accurate focusing than the other AF methods, the TV-AF method has the higher focusing accuracy and accordingly tends to require a longer time period to achieve focusing than the other AF methods. Also, when an object is taken with low contrast or in dark conditions, searching for an in-focus position is not necessarily performed accurately in the TV-AF method. If the search for an in-focus position is performed in a wrong direction in which a true in-focus position does not exist, it takes a long time to achieve focusing.
In the aforementioned hybrid AF method, focus adjustment is performed in an AF method other than the TV-AF method with lower accuracy of focusing than the TV-AF method. Thus, when a focus lens is first driven in the AF method other than the TV-AF method, the focus lens may pass an in-focus position at which the lens would stop in the TV-AF method. Thus, a video being taken involves unnatural focus changes in which focusing is once achieved, then slight blurring is caused, and thereafter, focusing is being achieved again by auto-focus in the TV-AF method.